docs(module-fragments): add instructions for create_agent, interactive, and remaining core tools

Three MCP tool groups were orphaned from the ambient CLAUDE.md context
because they shipped no `*.instructions.md` alongside their source.
Backfill them so the composer picks them up as fragments on next spawn:

- core.instructions.md: add `send_file` (artifact delivery, path relative
  to /workspace/agent/) and `add_reaction` (by `#N` id with emoji
  shortcode name).
- interactive.instructions.md: `ask_user_question` (blocking
  multiple-choice with selectedLabel/value option objects, 300s default
  timeout) and `send_card` (non-blocking structured render with
  fallbackText). Opens with a one-line framing of the contrast between
  the two.
- agents.instructions.md: `create_agent` with how-it-works, when-to-use
  (companions vs collaborators — persistent memory vs independent
  parallel work), when-NOT-to-use (short tasks should use the SDK `Agent`
  tool instead), and guidance for writing the seed instructions string.

No composer changes — scan in `src/claude-md-compose.ts` already picks up
any file matching `*.instructions.md` in the mcp-tools directory.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

container/agent-runner/src/mcp-tools/core.instructions.md

@@ -14,6 +14,14 @@ Use the `mcp__nanoclaw__send_message` tool to send a message while you're still
 
 **Outcomes, not play-by-play.** When the turn is done, the final message should be about the result, not a transcript of what you did.
 
+### Sending files (`send_file`)
+
+Use `mcp__nanoclaw__send_file({ path, text?, filename?, to? })` to deliver a file from your workspace. `path` is absolute or relative to `/workspace/agent/`; `filename` overrides the display name shown in chat (defaults to the file's basename); `text` is an optional accompanying message. Use this for artifacts you produce (charts, PDFs, generated images, reports) rather than dumping contents into chat.
+
+### Reacting to messages (`add_reaction`)
+
+Use `mcp__nanoclaw__add_reaction({ messageId, emoji })` to react to a specific inbound message by its `#N` id — pass `messageId` as an integer (e.g. `22`, not `"22"`). Good for lightweight acknowledgment (`eyes` = seen, `white_check_mark` = done) when a full reply would be noise. `emoji` is the shortcode name (e.g. `thumbs_up`, `heart`), not the raw character.
+
 ### Internal thoughts
 
 Wrap reasoning in `<internal>...</internal>` tags to mark it as scratchpad — logged but not sent.